1. Field of Invention
The present invention is related to novel devices and methods for spinal fixation, including in one aspect posterior spinal fixation, particularly posterior cervical fixation, utilizing a plate and locking screw system, designed for improved stability and safety over devices presently in use. The invention is also directed to novel devices and methods for occipital/spinal fixation, particularly fixation of the occiput to the posterior side of the cervical vertebrae, utilizing both the inventive device and method of posterior spinal fixation as well as other spinal fixation systems. The invention is also related to the use of orthopedic plates for stabilization of fractured bones, including skull bone following a surgical craniotomy or accidental fracture. Finally, the present invention is related to a novel locking screw system for use in orthopedic plate/screw fixation devices.
2. Description of Related Art
Occipital/cervical fixation:
Occipital/cervical fixation, although not a commonly performed procedure, is indicated for instability of the craniocervical junction resulting from trauma, degenerative disease, inflammatory arthritis, or malignancy.
Several techniques have been tried in recent years to improve the ease and reliability of occipital/cervical fixation, many of which are described by White and Panjabi in Clinical Biomechanics of the Spine, 2nd Ed. (1990). One common technique for posterior occipital/cervical fixation involves the use of wires. In particular, the occiput is fused to the upper cervical spine by a set of bilateral wires which are threaded through holes drilled into the C1 and C2 vertebrae and up through holes drilled into the occiput. Alternatively, occipital/cervical fixation may be accomplished by the use of Luque rods with intracranial and sublaminar wiring. While stability is good, these techniques are dangerous in that there is a serious risk of cerebrospinal fluid leak and/or bleeding from the sagittal sinus due to the placement of the holes in the occiput.
Another method of posterior occipital/cervical fixation is the use of a single plate molded according to the anatomy of the head and the cervical spine. At one end, the plate is screwed into the occiput, and at its other end the plate is screwed or wired to the vertebrae. One such plate is a Y-plate, as discussed by Grob, D. et al., "Posterior Occipitocervical Fusion," 16 Spine S17-S24, (1991). Here, the "arms" of the plate are attached to the C1 and C2 vertebrae by transarticular screws while the proximal end of the plate is attached to the occiput by screws. The C1-C2 fusion is accomplished by use of a technique developed by Magerl wherein two non-locking screws connect the two vertebrae, at an upward 30.degree. angle, through the top of the C2 lamina, across the C1-C2 articulation, and into the lateral mass of C1.
Alternatively, bilateral fixation plates can be used for subaxial (i.e. vertebrae below C1) fusion, comprising the use of two reconstruction plates. According to Grob and associates [(16 Spine S17 (1991)], the plates are fixed to the occiput by screws through the skull, and then they are fixed distally to the lateral masses of the subaxial vertebrae. To attach the plates to the vertebrae, the drilling of the holes is directed cranially and parallel to the joint surface and 20.degree.-30.degree. laterally to avoid damage to the vertebral artery and nerve root.
The use of internal fixation to the occiput is made challenging by the extreme variability in the thickness of the skull itself and the rather major penalties which can complicate these procedures. The current procedures generally known in the industry for occipital/spinal fixation involve insertion of screws and/or intracranial wires through the very thin bone of the cerebellar fossa. Resulting complications of occipital fixation thus include the creation of dural leaks, direct trauma to the brain, penetration of the large intracranial venous sinuses, and iatrogenic meningitis. Other complications associated with any internal fixation device, such as hardware loosening, hardware pull out, and hardware fracture, for example, can also occur.
Consequently, it is desirable to have a method of occipital/spinal fixation wherein the occipital fixation device is placed in an optimal location on the skull, ideally the thickest bone, for increased safety as well as optimal stability. Such a method is best accomplished by a modular system whereby the surgeon is able to mount a plate to a desired location on the occiput, and then attach a separate spinal fixation device, such as rod or plate, for example, posteriorly to the desired vertebrae. The superior end of the spinal fixation device can then be adapted to attach to the plate, or a third device can be used to connect the plate to the spinal fixation device.
Posterior Spinal Fixation:
The use of segmental screw fixation has revolutionized the practice of thoracic and lumbar spinal surgery. Implants with screws which lock rigidly to the plate are extremely stable and allow the surgeon considerable freedom in performing very aggressive destabilizing surgery. This is often to the patient's advantage in cases of extreme spinal instability, fracture, or tumor. Examples of such implants include the Kaneda device (vended by AcroMed, for example.) for anterior thoracic and lumbar use, for example, and the Cotrel-Dubousset (manufactured by Sofamor) and Texas Scottish Rite (vended by Danek) pedicle screw configuration methods for the posterior thoracic and lumbar spine.
A current method of posterior cervical fixation involves C1-C2 sublaminar posterior wiring used in combination with Luque rods. While this method works fairly well to immobilize the spine, there is an increased risk of spinal cord injury due to the necessity of passing multiple sublaminar wires through the cervical spinal canal. Other posterior spinal fixation methods include spinous process wires and facet wires typically used in the lower cervical spine, Halifax clamps which clamp together the lamina of the C1 and C2 vertebrae, and hook plates placed underneath the lamina of the lower vertebrae involved and fixed to the articular process of the upper vertebrae by a screw (as described by Jeanneret, B. et al., "Posterior Stabilization of the Cervical spine with Hook Plates," 16 Spine S56-S63 (1991)).
Another method of posterior spinal fixation involves the use of screws and/or plates. Anderson and associates in their article "Posterior Cervical Arthrodesis with AO Reconstruction Plates and Bone Graft" (16 Spine S72-S79 (1991)) discuss fixation of the cervical spine by the use of these non-locking, lateral mass plates which are attached to the vertebrae by screws inserted into the vertebrae at a 30.degree.-40.degree. cranial (i.e. upward) direction. Each screw attaches one vertebra, such that the vertebrae are connected to each other only by means of the plate. The plates are malleable and have different hole spacings to accommodate variations in patient size and anatomic levels to be fused.
A similar technique for posterior cervical fixation using lateral mass screws and plates was originally described by Roy-Camille, et al. (Recent Advances in Orthopaedics, B. McKibbin, Ed., p.51-87 (Churchill-Livingston, Edinburgh 1979) and is explained in greater detail by Nazarian and Louis (16 Spine S64-71 (1991)). This technique also involves placing a screw into each individual lateral mass and then attaching or connecting the vertebrae together by means of a non-locking plate. The direction of screw placement is different from the Magerl method, discussed below; however, the purpose of both methods is to obtain bicortical purchase through the lateral mass of a single vertebra without penetration of the facet joint.
All of the foregoing methods are examples of segmental fixation; however, they do not rigidly attach the screws to the plate. All of these implants, for example, may have problems when used on a grossly unstable spine, such as that created by a complete cervical vertebrectomy. The non-locking lateral mass plates, such as the reconstruction plate vended, for example, by AO, also require an extraordinary degree of technical expertise by the surgeon. Acquiring bicortical purchase with these lateral mass screws requires that the screw be placed "freehandedly" to within millimeters of the vertebral artery and the exiting cervical nerve root. Injury to either of these structures may have devastating consequences for the patient.
The Magerl method, as discussed, for example, by Montesano, et al. ["Biomechanics of Cervical Spine Internal Fixation" (16 Spine S10-S16 (1991)], involves the attachment of the anatomically unique C1 and C2 vertebrae by two non-locking screws. The screws are engaged at an upward angle by insertion from the tip of the lamina of C2, across the C1-C2 articulation, and into the lateral mass of C1. The screw length is about 18-20 mm, and the angle of purchase is 30.degree. in an upward (i.e. cranial) direction. This method of fixation, however, is a relatively dangerous procedure in that there is a great risk of damage to the exiting cervical nerve root and the vertebral artery, similar to the risks associated with the use of non-locking lateral mass plates discussed above.
Another means of obtaining posterior internal fixation of the spine is the use of screws to transfix the facet joints. This method, as applied to the lumbar spine, has been described by M. Heggeness and S. Esses, 16 Spine S266-S269 (1990). This method has also been applied in the anatomically unique (and complex) C1-C2 level of the spine, as described by Montesano, et al., 16 Spine S10-S16 (1991).
In light of the problems associated with existing devices and methods for posterior spinal fixation, in particular posterior cervical fixation, it is desirable to have a device and method that affords adequate immobilization of the spine, including a grossly unstable spine, without posing a significant threat to the vertebral artery or cervical nerve root.
Plate/locking screw systems for spinal fixation:
Presently, there are very few locking screw devices on the market for use in spinal fixation. Perhaps the most commonly used locking screw is that used in the cervical locking plate manufactured by AO/Synthes ("CSLP"). This screw consists of a cylindrical screw head, drilled coaxially with the long axis of the screw shaft. That is, there is a smaller set screw in the head. The walls of the head are also slotted, so placement of the set screw expands the walls of the head, locking it into the plate. Aspects of this screw include minimal risk of screw pull-out. This method, however, has the disadvantage that it is easy to accidentally misplace or drop the smaller set screw into the surgical wound during use.
Another locking screw system is the "ORION" plate (sold by Danek), which is also a "two-screw" system. In this system, one larger screw is placed into the hole of the locking plate and then positioned into the bone like other screw/plate devices. However, a second screw with a much larger head is inserted into a smaller hole adjacent to the first hole containing the larger screw. Once the larger screw is in the proper position in the bone, the smaller screw is inserted into the plate as described above and torqued down until a portion of the head of the smaller screw covers a portion of the head of the larger screw, thus preventing the larger screw from backing out. This method also requires the use of an additional small "locking screw" which may be dropped or misplaced. The use of this type of locking screw also adds bulk to the implant and may impinge the visceral structures of the patient.
The AMS Company manufactures another type of locking screw/plate system utilizing a single screw. The screw comprises two sets of threads on the longitudinal piercing portion of the screw. The set of threads immediately subjacent to the screw head are designed to engage into a threaded hole contained within the locking plate. The lower set of threads function to engage the bone. Thus in this system, when the screw is torqued to engage the lower set of threads into the bones, the screw is simultaneously "locked" into the plate via threadable engagement of the upper threads of the screw with the hole in the locking plate. While this system is advantageous in that only one screw is required, the disadvantage of this system is that the screw placement function and the locking function are combined, thereby making it difficult for the surgeon to "feel" where he or she may be in the bone being penetrated. The quality of bone/screw purchase is thus more difficult for the surgeon to assess.
It is therefore desirable to have a locking plate/screw system requiring only one locking screw, but where the locking function and the bone placement function are separate for safer and easier bone purchase. It is also desirable to minimize the number of components in the locking mechanism.